Modern electronics are ubiquitous in healthcare. For example, monitoring devices often include electronic components and algorithms to sense, measure, and monitor living beings. Monitoring equipment can measure vital signs such as heart rate, oxygen level in the blood, respiration rate, and so on. Not only are vital signs monitoring devices used in the clinical setting, but such devices are also used often in sports equipment and consumer electronics.
One important measurement performed by many of the monitoring equipment is heart rate, typically measured in beats per minute (BPM). Athletes use heart rate monitors to get immediate feedback on a workout, while health care professionals use heart rate monitors to monitor the health of a patient. Many solutions for measuring heart rate are available on the market today. For instance, electronic heart rate monitors can be found in the form of chest straps and watches. One technique often employed in wearable heart rate monitors is an optical measurement technique known as photoplethysmography (PPG).
In a PPG-based heart rate monitor, a signal indicative of a heart rate of a living being (said signal referred to in the following as a “PPG signal” or as a “heartbeat signal”) is obtained by illuminating the skin (using a light source) of a living being, and measuring changes in light absorption (using an optical sensor). The principle of operation of PPG-based heart rate monitors are based on recognition that a heart rate can be measured passively or indirectly based on changes in light absorption in the skin of a living being as blood is pushed through the arteries. Changes in blood volume as blood is pumped through the arteries results in a variation in the amount of received light, which is translated into electrical pulses by an optical sensor. The pulses in the PPG signal can then be used in extracting a heart rate of the living being. Application of PPG measurements is not limited to measuring heart rate. In fact, PPG may be used to evaluate various other vital signs of living beings, such as blood pressure or perfusion.
Unfortunately, PPG-based electronic monitors are often not very accurate, largely due to a high amount of noise present in the signals provided by the sensors of these monitors. Noise is often caused by motion of a living being during the measurements and/or by movement of a measuring device with respect to the living being during the measurement. Such a noisy environment makes it difficult for the PPG-based monitors to output a consistently accurate vital sign reading and may lead to an irregular, inaccurate or even missing readout of the vital signs. One known technique for reducing motion-related artifacts (i.e., signal contributions due to motion of a living being) in PPG signals includes processing accelerometer readings that measure movement. Accelerometer measurements may help with filtering out some of the motion-related artifacts from PPG signals, but not always and not always to the desired degree. Therefore, improvements with respect to devices and methods for reducing motion-related artifacts from PPG signals used to determine various vital signs of living beings would be desirable.